This invention relates to an apparatus for measuring the spectral transmissivity of optical fiber.
Apparatus for measuring the transmissivity of optical fiber, either dedicated for the purpose or standardized as a peripheral of a spectro-photometer, have not been commercially available. It has been customary for the user, therefore, to either have an apparatus specially made or design one for himself. There has not been even a standard method of measuring the transmissivity of optical fiber. Apparatus thus designed and used by individual users for measuring the transmissivity of optical fiber are structured basically as shown in FIG. 3 or 4.
In general, transmissivity of a given material is determined by measuring the light value of an incoming monochromatic test beam with a sample placed in its optical path and by removing it therefrom to obtain the so-called 100% transmissivity. According to the prior art method illustrated in FIG. 3, a mask M, having an aperture with diameter smaller than the bundle diameter of an optical fiber sample 3 to be measured, is placed within the optical path of an outgoing beam from a spectro-photometer, and an integrating sphere 2 with a light-receiving window is set at a certain distance from this mask M such that the outgoing beam from the spectro-photometer can be introduced thereinto. For the measurement of the 100% transmissivity, the space between the mask M and the integrating sphere 2 is left empty as shown in FIG. 3A. For determining the transmissivity of the optical fiber sample 3, the light-receiving end of the sample 3 is placed close to the opening in the mask M and its light-emitting end is place closed to the light-receiving window of the integrating sphere 2, as shown in FIG. 3B. The distance between the mask M and the integrating sphere 2, therefore, should be sufficiently large for placing the sample 3 in between.
According to the method illustrated in FIG. 4, the entire outgoing beam from a spectro-photometer is made incident directly into an integrating sphere 2, as shown in FIG. 4A, for the measurement of 100% transmissivity. For the measurement of transmissivity of an optical fiber sample 3, the same outgoing beam from the spectro-photometer is caused to converge into the light-receiving end of the sample 3 by using a lens L or the like, as shown in FIG. 4B. The light emitting end of the sample 3 is placed close to the light-receiving window of the integrating sphere 2, as shown in FIG. 4B. It now goes without saying, regarding the prior art methods described above, that an identical beam must be directed, at one time, into the integrating sphere 2 for the measurement of the 100% transmissivity and, at another time, into the sample 3 of which the transmissivity is to be measured. When a spectro-photometer is used, the sample is usually placed inside a sample chamber. When the transmissivity of optical fiber is to be measured, however, since the optical fiber cannot be placed inside the sample chamber, it is necessary to set is outside the sample chamber and to guide the light out of the chamber. In this situation, the diameter of the outgoing beam (indicated by letter F in FIGS. 3A and 4A) from a spectroscope outside the sample chamber is greater than the bundle diameter (about 1-3mm) of the optical fiber. In the example of FIG. 3, the mask M is used for the purpose of providing and introducing a beam of the same diameter into the integrating sphere 2 for the measurement of 100% transmissivity and into the optical fiber 3 for the measurement of its transmissivity. In the example of FIG. 4, the entire outgoing beam from the spectroscope is made incident into the integrating sphere, but a convergent lens L is necessary to make it totally introduced into optical fiber sample 3. By the method of FIG. 3, the signal-to-noise ratio is not good because only a limited portion of the outgoing beam from the spectro-photometer is utilized and hence the available quantity of light is reduced. The use efficiency of light is better with the method of FIG. 4, but since the lens L is inserted only when the transmissivity of the sample is measured, what is actually measured is the combined transmissivity of both the lens L and the optical fiber sample 3. In other words, the method according to FIG. 4 has the disadvantage of reduced accuracy.
As described in Japanese Patent Application 2-262578 and illustrated in FIG. 5, the present inventor proposed a new method whereby a converging lens L is inserted for the measurement of both a sample (FIG. 5A) and the 100% transmissivity (FIG. 5B). The same lens L can be placed interchangeably at two different positions along the optical path of the outgoing beam from the spectro-photometer. By this method, the difference in optical path length between the two measurements does not include the transmissivity of the lens L, and the same amount of light can be made incident into the integrating sphere for the measurement of the 100% transmissivity and into a sample for the measurement of its transmissivity. Thus, both the signal-to-noise ratio and the accuracy of measurement can be improved. By this method, however, the optical fiber sample must be placed with its end portions positioned in a collinear relationship and hence a relatively large space is required between the mask M and the integrating sphere 2. Another disadvantage of this method is that the converging lens L must be moved between the two measurements.
It is therefore an object of the present invention to provide an apparatus, serving as a peripheral of a spectro-photometer, for measuring the true spectral transmissivity, in principle, of optical fiber by making use of light with improved efficiency.